This invention relates to an unsaturated polyester resin composition which is particularly useful in the manufacture of electrical laminates. The term "electrical laminates" as used herein refers to unclad insulation boards for mounting various electrical and electronic parts thereon as well as one or both sided metal clad laminates for use in the manufacture of printed circuit boards (PCBs). They are normally in the form of a flat sheet having a thickness of about 0.5-5 mm.
Electrical laminates made from unsaturated polyester resins are now commercially available and consumed in large quantities. These laminates are produced by pre-treating a cellulosic substrate with a melamine-type resin, impregnating the substrate with a liquid unsaturated polyester resin, laminating a plurality of the resin-impregnated substrates, and curing the laminate. A continuous process for the production thereof is disclosed in U.S. Pat. No. 4,372,800 assigned to the assignee of this application.
Unsaturated polyester resins in general find a variety of uses owing to their excellent mechanical properties and molding workability with various reinforcing materials. For use in the manufacture of PCBs, they must comply with high standard requirements for properties in terms of punching property, solder dip resistance, impact strength and the like. Needs exist for resins having an improved impact strength without deteriorating heat resistance in the field of large FRP products. These property requirements have become more stringent as automated processing of PCBs has become more popular and parts are packaged thereon more densely. However, these requirements are often incompatible. For example, it has been difficult to improve both the heat resistance and impact strength simultaneously or to improve all of the low temperature-punchability, solder dip resistance and impact strength while keeping a good balance among them.
Attempts have been made to improve the impact strength by modifying unsaturated polyester resins with flexible resins, plasticizers, liquid butadiene-based polymers, cross-linked diene- or acrylic-based rubbers and the like. However, the incorporation of these modifiers is not satisfactory in that they tend to deteriorate heat resistance, chemical resistance, stability against phase separation, substrate-impregnating properties and other properties.
It is, therefore, a major object of the present invention to overcome the above-discussed problems. Other objects and advantages thereof will become apparent as the description proceeds.